A mass vaccination event in Raleigh and clinics in Hillsborough and Chapel Hill stopped administering the single-shot Johnson & Johnson coronavirus vaccine Thursday after at least 26 people experienced adverse reactions, according to state health officials.

Wake County gave out more than 2,300 shots of the Johnson & Johnson vaccine at the PNC Arena before they decided not to give more of the vaccines.

"Wake County EMS evaluated 18 total patients. Fourteen of those people had minor reactions and were treated on site. Four others were transported to area hospitals were they are being evaluated and are expected to be released," Wake County public health officials said.

The county said officials consulted the state Department of Health and Human Services. The county said the Centers for Disease Control and Prevention is analyzing the vaccine batch and could give a response on what to do within hours.

"We know it can be alarming to hear about or see people having reactions to vaccination - this is why we closely monitor those we vaccinate in case of reaction," Wake County Public Health's Medical Director Kim McDonald said in a statement.

"Right now we are working with NCDHHS and the CDC to further evaluate the situation to assure everyone is confident in the continued safety of our vaccine operations," she said.

The U.S. Centers for Disease Control and Prevention noted that reactions like fainting are not uncommon after someone is vaccinated, though it is reviewing reports of adverse reactions in North Carolina and three other states.

On Wednesday, a vaccination event in Colorado was halted because of similar concerns over doses of the Johnson & Johnson vaccine, according to CBS Denver.

People who still had appointments with Wake County to get the Johnson & Johnson vaccine were offered the Pfizer vaccine instead, the department said.

A pair of clinics in Hillsborough and Chapel Hill also elected to stop offering the Johnson & Johnson vaccine and are in the process of rescheduling 350 appointments for people who were set to receive their single-dose J&J shot on Friday.

CDC spokeswoman Kristen Nordlund said it is aware of adverse reactions in some people who received the vaccine shots in Iowa, Colorado, Georgia and North Carolina. Those reactions include dizziness, light headedness, feeling faint and rapid breathing.

She said the CDC is working with state and local officials to evaluate the issues and has performed vaccine lot analyisis and not found reasons for concern. The CDC is not telling health departments to stop vaccinations.

“COVID-19 vaccines are safe and effective,” she said. “Many people don’t have any side effects after COVID-19 vaccines, but some people will have pain or swelling at the injection site or fever, chills, or a headache. These typically don’t last long and are signs that your body is building protection.”

David Wohl, an infectious disease expert at UNC Health who oversees the UNC vaccination sites, said between eight and 14 of the roughly 1,250 J&J vaccine recipients it injected on Thursday fainted after receiving the shot, though nobody was taken to a hospital.

Because of the timing of the day in which the adverse reactions started occurring, Wohl said UNC Health completed all shots for people who had appointments on Thursday at the two sites.

While his department is still working to analyze the source of the problem, he does not believe most of the cases stem from allergic reactions to the vaccine.

“I’m just concerned that if we have as many people fainting as we do, we need to understand why,” Wohl said. “Everyone that I’ve seen really doesn’t have an underlying medical history that’s of concern, but many of them do report having fainted previously.”

Johnson & Johnson, the vaccine’s manufacturer, said in a statement that “there is no greater priority than the safety and well-being of the people we serve. When we receive reports of adverse events in individuals receiving our medicines and vaccines, we collect necessary information and carefully assess the events.”